Anisotropic Conductive Elastomer (ACE) is a composite of conductive metal elements in an elastomeric matrix that is normally constructed such that it conducts along one axis only. In general, ACE is made to conduct through its thickness. One form of ACE material is made by mixing magnetic particles with a liquid resin, forming the mix into a continuous sheet, and curing the sheet in the presence of a magnetic field. This results in the particles forming a large number of closely spaced columns through the sheet thickness. The columns are electrically conductive. The resulting structure has the unique property of being both flexible and anisotropically conductive. These properties provide for an enabling interconnection medium which, when combined with other technologies, make it possible to realize new interconnect capabilities.
ACE material is married with flexible circuits to provide more dynamic range to an ACE interconnect. The flexible circuit consists of an electrically-insulating material such as polyimide, with opposing conductive pads on the surfaces. The pads are vertically interconnected by plated-through holes. Mounting such a flex circuit to ACE material provides more vertical compliance to the ACE material. This allows the ACE material to be used in assemblies that are not flat, such as circuit boards with solder mask, in which the circuit board pads are lower than the top of the solder mask, thus creating a small well around each pad and into which the ACE material-based interconnect must protrude in order to make electrical contact with the pads.
BGA (ball grid array) devices are electronic components with solder balls placed in a grid and used for final installation (through a solder reflow process) on a printed circuit board. There are other types of electronic packages in which the invention is relevant, including Land Grid Arrays (LGA) that are similar to BGAs, except that the solder is not ball shaped, but rather is formed into a square pad with minimal height. Also, Column Grid Arrays (CGA) have solder pins in place of the ball or pad. Factors such as cost, environment and population density determine which geometry is used.
It is important for the surface of the solder ball, pad or pin that touches the circuit pads be as free of defects as possible. Deformation of the solder ball, especially in the apex area, can result in a poor or a non-functioning electrical connection. The BGA device is typically tested using a custom socket assembly that utilizes spring-loaded contacts. These contacts may have a serrated face that allows tangential contact with individual solder balls.
Although a serrated contact face provides an excellent electrical contact, the overall length of the pin creates a higher electrical inductance for the contact pin assembly as compared to the short path length provided by the ACE material. As microprocessor speeds increase, inductance hampers performance. Some manufacturers have seen this degradation at bandwidths as low as 500 MHz.